| Carbon nanofiber is a new type of nanofiber.Because of it has many precursors,such as polyacrylonitrile?PAN?,asphalt,and rayon,it also has excellent properties,such as high modulus,low density,high specific performance,no creep,and non-oxidation.It has been widely concerned about its resistance to ultra-high temperature and good fatigue resistance in the environment.Polyacrylonitrile?PAN?is a kind of carbon nanofiber precursor material with better performance,while cellulose nanofibrils?CNF?is a new type of inorganic nano reinforcement material.Combining the two with electrospinning is a new idea to reinforce carbon nanofibers.In this research,electrospinning was used to prepare polyacrylonitrile?PAN?/nanocellulose?CNF?composite nanofibers,and then carbon nanofibers with significantly improved properties were prepared by high-temperature carbonization,thereby expanding the application fields of PAN and CNF.The research mainly include:?1?Optimization of PAN-based carbon nanofiber preparation process.A polyacrylonitrile electrospinning solution was prepared using polyacrylonitrile as the raw material and N,N-dimethylformamide as a solvent,and a polyacrylonitrile nanofiber membrane was prepared by electrostatic spinning technology,and then subjected to stabilization and high-temperature carbonization to prepare carbon nanofibers with a certain strength were obtained,and their properties and morphology were characterized.According to the strength of the nanofiber membrane,the preparation process parameters were optimized.The results show that the optimal preparation process of carbon nanofibers is:a spinning solution concentration of 12wt%,a propulsion speed of 15mm/min,a spinning distance of 15 cm,a spinning voltage of 20 kv,a spinning time of 180 min,and a stabilization temperature of280°C.,The time is 10 min,and the carbonization temperature is 1200°C.The breaking strength of the PAN nanofibers prepared by the optimized process before carbonization was 55 Kpa,and the breaking strength after carbonization was 13.1Mpa.?2?Optimization of PAN/CNF carbon nanofiber preparation process.In order to improve the breaking strength of PAN nanofiber membrane,hydrophobic modified nanocellulose?CNF?was added to PAN spinning solution,and PAN/CNF composite fiber membrane was prepared by electrostatic spinning technology.It was pre-oxidized and carbonized to obtain carbon nanofibers with certain electrical and mechanical properties.The effects of CNF length on the morphology and properties of spinning solutions and PAN nanofiber membranes were systematically studied using rheological performance tests,conductivity tests,SEM,TG,DSC,contact angle tests,and mechanical tests.The results show that CNF is distributed between the PAN fibers,which strengthens the interaction force between the fibers,thereby further increasing the breaking strength of the PAN/CNF fiber membrane.Due to the strong polar interaction between the nitrile group and the hydroxyl group formed between PAN and CNF,the viscosity and conductivity of the spinning solution increase with the increase of the length of CNF.The size distribution of PAN/CNF nanofibers is uniform,without obvious beading and agglomeration,the average diameter of PAN/CNF nanofibers gradually decreases as the size of CNF increases.In addition,the addition of CNF can improve the thermal stability and mechanical properties of PAN.The addition of CNF can improve the mechanical properties and electrical conductivity of PAN carbon nanofibers.The breaking strength was increased from 55Kpa to 79.1 MPa,and the resistance is reduced from 40 K?to about 10 K?.The addition of CNF can increase the carbonization rate and reduce the carbonization temperature of PAN nanofibers.Therefore,CNF can be used as an effective reinforcing material for PAN-based carbon nanofibers.?3?Study on carbonation mechanism of PAN/CNF carbon nanofibers.This article analyzes the physical and chemical properties of nanofibers at different stages,such as surface morphology,functional group structure,weight loss rate,thermal stability,and electrical conductivity,etc.Carbonation reaction mechanism.As a result,at the pre-oxidation stage,for the PAN/CNF nanofiber membrane,the PAN polymer chain undergoes a self-cyclization reaction,and the CNF mainly undergoes a dehydration reaction.During the self-cyclization of the PAN chain,it interacts with the-CH bond of the CNF,thereby inserting a small amount of CNF into the molecular chain of the PAN,and finally forms a PAN/CNF interaction structure.In the carbonization stage,the polymer chain of PAN/CNF nanofibers is broken from a depolymerized structure to a graphite-like layer structure after the polymer chain is broken.In the PAN ring structure,the-CH bond between the PAN molecular chains interacts at a high temperature to form a flake-like hexagonal graphite structure.And CNF at this carbonization stage will aggregate into monosaccharide derivatives.The intermediate then forms a heterocyclic aromatic structure that releases a gas containing non-carbon atoms and interacts with the PAN molecule to form a layered ring structure.After heat treatment above 1200°C,the carbon residue is transformed into a more ordered carbon structure in an N2atmosphere.?4?Study on the surface modification of PAN/CNF carbon nanofibers.On the basis of carbonization,the physical and chemical modification methods of PAN/CNF composite fiber membranes are compared to further improve the mechanical properties of carbon nanofibers.The physical modification is to apply a certain stretch ratio during the carbonization process.The chemical modification is to use an acetic acid solution to impregnate the fiber membrane prepared by electrospinning,and then perform pre-oxidation and carbonization.The results show that the breaking strength of PAN/CNF carbon nanofibers increases first and then decreases with the increase of the stretching ratio,reaching a maximum of 85 Mpa.Acetic acid can reduce the reaction energy of the fiber membrane during stabilization and carbonization carbon nanofibers with more stable fiber shape. |
PDF Full Text Request